Simultaneous analysis of downlink beacon dynamics and sky brightness temperature. Part I. Remote sensing of a slant path

Abstract

This paper investigates the remote sensing of height and displacement velocity of turbulent clouds, composed mainly of liquid water, using a combination of simultaneous radiometric and geostationary satellite beacon measurements. Because a radiometer integrates the sky brightness temperature distribution observed within the solid angle subtended by the antenna beamwidth, the paper shows how this process can be modeled as a low-pass filter using the small angle approximation and the frozen-in hypothesis. The 3-dB cutoff frequency of that filter is used in conjunction with the Fresnel frequency of the amplitude scintillations power spectrum to derive the cross-path wind speed and the height of the turbulent layer/cloud. In order to quantify the 3-dB cutoff frequency of the filter-radiometer, a dual aperture radiometer was constructed and was used in conjunction with a ground station receiver monitoring the 39.5-GHz beacon transmissions of ITALSAT F1 and measuring amplitude scintillations. The experimental results derived on cloud heights are then compared with the vertical profiles obtained by a 94-GHz radar, indicating agreement except in those cases where the thickness of the cloud structure becomes important. This aspect is then discussed in the paper and a radiometric definition is given for the cloud height. Finally, the important aspect of instrumentation is described including the effect of the radiation pattern of a radiometer antenna upon the cross correlation and cross spectrum between two coaxial measured antenna temperatures.